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Here is wikipedia page containing a list of plants used in herbal medicine. One might first want to argue that many of them actually do not have any medicinal/beneficial effect on heatlth. I think we have evidences for quite a lot of species that they actually mimic drugs. For example Filipendula ulmaria is rich in acetylsalicylic acid (aspirin). So I hope, you'll agree to assume that there are quite a lot of plant species that contains substances that are usable in medicine. If there are many of them, my question is:

Evolutionarily speaking, why are there so many drug-like plants?

At first, I woud say that an alllele for producing a substance which is beneficial for a predator should not get fixed in the population! so why are they so numerous to have medicinal properties?

Here are some (intuitive, unclear or far-fetched, non-exhaustive and non-exclusive) hypotheses I can think of:

  1. Because plants want their seeds to get ingested in order to propagate

  2. Is it because there are so many substances that affect our homeostasy that many plants are toxic and many are healthy just by chance. Containing healthy substances are not adaptations but by-product of evolution.

  3. The substances that are beneficial for us actually evolved to repulse predators. These substances are toxic at high dosage and therefore are efficient against predators that are small or eat a lot of plants (herbivorous). These substances at low dosage might actually have a benefical effect. For example a substance that make blood thicker is very toxic except if you eat just a bit of it while bleeding.

  4. We (primate or whatever taxon you want to consider) evolved in order to get advantage of the surrounding envirronment. Sbstances that were neutral became beneficial. The advantage of being sensitive to various products cause that, by chosing our food source, we can heal. Therefore, by evolving sentivity to various substances, our behaviour can act as a reinforcement to immunity (and other anti-illness system). If this is true, we might expect that the frequency of presence of a plant species affect the probability that out homeostasy get affected when eating it.

  5. Lineage selection. Lineages that produce substances that are active in predators body in a way or another are undergoing many various selection forces because of these substances. Therefore they get a higher speciation rate than other lineages.

  6. Keeping some non-herbivorous species in large population size (by helping them) is the best way to keep herbivorous species in small population (because of territory competition, predators relationships, etc... )

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Thks for you answers! I think, you both roughly speaking picked the hyopthesis 3. Let's take again the example of salicilic acid. This acid has an important pharmacological effect but is not directly used by the plant to defense itself. It is nothing more than a hormone (and not sexondary metabolites such as nicotins, proanthocyanidines, menthols, cyanides, etc...). –  Remi.b Aug 3 '13 at 9:01
So why did the Filipendula ulmaria evolve a hormone which has pharmacological effect on human (and all mammals I guess, and maybe even all deuterostome or more)? hypothesis 3 does not provide an explanation for chemicals that are not involved in toxicity. –  Remi.b Aug 3 '13 at 9:03
I think you may be kind of falling in the trap of "if it is there there is a reason"... How many other chemicals are present in the same plant that do not have an action on humans? Just because one specific compound interacts with a protein in humans it does not mean it evolved for that reason. You should consider that s.a. has a very simple aromatic structure, and it is not surprising that it can interact with other proteins than the plants endogenous ones. In any case, when taken in large amounts, s.a. is indeed toxic, so in the specific case, explanation #3 would still stand. –  nico Aug 3 '13 at 13:26
Well, I'm not falling in this trap. It would mean that a combination of the hypotheses 2 and 3 are responsible for the presence of these chemicals. –  Remi.b Aug 3 '13 at 21:44
@Remi.b - Just to clarify, a secondary metabolite is not necessarily used strictly for defense, it is simply a compound that is not detrimental to the plant if not present. For example, some secondary metabolites are pigments, some are hormones, and some involved in structural support (such as lignins, derived from phenolic compounds). On an unrelated note, a "sexondary metabolite" (see your first comment) is presumably a Freudian typo. –  J. Alfred Prufrock Aug 4 '13 at 4:19
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2 Answers

up vote 5 down vote accepted

Perhaps the question may also be phrased, "Why is it common for plants to produce chemicals that possess pharmacological or toxicological effects in man and animals?", and to that question it is often reasoned that plants, being sessile and otherwise defenceless food sources for predators, produce compounds that affect the physiology of animals in such a way as to make it not beneficial for a predator to consume them, which is close to your 3rd hypothesis. Remember that pharmacological benefit is largely anthropocentric, and relative at that. For instance, a plant compound that lowers blood pressure could be used as treatment of hypertension, but if the plant were to be consumed in its native state in appreciable quantity, it could put the eater into shock, cardiac arrest, etc.

Most pharmacologically active compound from plants comprise a class of compounds known as secondary metabolites: metabolic intermediates that are not appreciably involved with functions of growth, respiration, etc. (often interpreted as evidence for a defence mechanism).

Try this wikipedia article and see what you think: http://en.wikipedia.org/wiki/Plant_defense_against_herbivory

The major classes of secondary metabolites in plants are: terpenes, which are polyisoprenoids, phenolic compounds, including the polyphenolic compounds, which are a popular study in natural products research, and alkaloids (along with other nitrogenous compounds).

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+1 for a good answer (and a great username, good to see a scientist who likes poetry :) ). –  terdon Aug 3 '13 at 17:20
Nice to see that the wikipedia article cites the Raven and Ehrlich paper (which hypothesizes a coevolution of plant chemical defenses and insect antagonism). –  Oreotrephes Aug 29 '13 at 0:38
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Firstly, most plants or other natural medicinals existed way before we knew about them; it's not that plants mimic drugs, its that drugs mimic plants.

To answer your question succinctly, compounds that we can use for our own medical benefit often do other things, we just adapt them for our own purposes. Penicillin is probably the best example. It didn't evolve to allow humans to fight bacteria, it arose in fungi and helped fungi attack bacteria; we just appropriated it. Similar with plant compounds like taxol (technically fungi again). Another good example can be found in my recent answer about snake venom. While a lot of those compounds are toxic to humans, they are very similar or even identical to compounds that in different doses can be beneficial.

Substances like capsaicin (aka spiciness) and caffeine are actually irritants that plants use to prevent predation, but we, in certain uses and doses, go wild for them. Same for the substances in mustard, ginger, or pepper. Imagine all the plant or animal chemicals we don't use!

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